The Effect of Particle Shape on the Amplitude of scattered light for a sizing instrument

The relationship between size and intensity of laser light scattered from coal particles, glass beads and calibration pinholes in the size ranges 10–140 m̈m, 17–240 m̈m and 5–200 m̈m respectively has been measured. The purposes were to determine the effect of shape of non‐spherical particles and beads by varying three parameters in the collection optics. These were: the angle, θ c , between the axis of the collection optics and that of the incident laser beam; the aperture of the f/3.66 collection lens; and the diameter of the spatial filter on the photodetector. A spatial filter diameter of 0.5 mm was used on the detector as a compromise between image vignetting and reducing the size of the effective measuring volume of the instrument. Due to refraction, and particularly for diameters larger than about 80 m̈m, the beads scattered more light than the pinholes or the coal particles. A mask, introduced to limit the collection lens aperture to angles less than 5.3°, reduced the intensity collected from the beads, while the response curves for the pinholes and coal particles were unaffected. Non‐spherical beads larger than about 60 m̈m with aspect ratios up to 1.8 resulted in departure from the response curve, established from the calibration pinholes, by up to a factor of two. The aspect ratio was defined by the maximum and minimum projected lengths of the image of the scatterer. The response curves for beads and coal particles approached that of the pinholes as θ c was changed from 2.5° to 1.4° and as the angular position of the mask limiting the aperture of the collection lens nearest to the incident beam axis, θ i , was reduced from 2.9° to 0.9°. The best results were obtained for θ i and θ c equal to 0.9° and 1.4° respectively, for which the precision and accuracy were 9 m̈m and + 5 m̈m for nominally spherical glass beads and 5 m̈m and −1 m̈m for the coal particles for diameters up to 60 m̈m and aspect ratio up to 2.5.